Acetic acid is one of basic chemicals, and is important in the petrochemical industry, the industry of polymer chemistry, the organic chemical industry, and the pharmaceutical and agricultural manufacturing industry.
Acetic acid is produced by a variety of processes, and among others, a process for producing acetic acid from methanol and carbon monoxide is industrially superior to other processes.
In recent years, as an improved process for producing acetic acid from methanol and carbon monoxide, it has been proposed to improve productivity of acetic acid and reduce generation of by-products by reducing a water content in a reaction fluid. However, in the case where the water content in the reaction fluid is reduced, a rhodium catalyst is easy to precipitate, and it is unfavorable in catalyst activity or catalyst stability, and in addition, the reaction rate is easy to decrease. Therefore, it has been investigated to add an iodide salt such as lithium iodide to a reaction system, or to prevent the undesirable effects of lowering of a water content in a reaction fluid by using a specific catalyst system. For example, Japanese Patent Application Laid-Open No. 54334/1985 (JP-60-54334A) (Patent Document 1) discloses a process which comprises allowing an alcohol or a derivative thereof to react with carbon monoxide in the presence of water and a catalyst system containing a rhodium component and an alkyl halide component to produce a carboxylic acid, wherein, in the liquid-phase carbonylation reaction, an iodide is added to the catalyst system in order to maintain an iodine ion concentration of not less than 0.3 mol/l in the carbonylation reaction fluid. Japanese Patent Application Laid-Open No. 239434/1985 (JP-60-239434A) (Patent Document 2) discloses a process for producing a lower carboxylic acid which comprises allowing a lower alcohol to react with carbon monoxide in a reaction medium containing a rhodium catalyst, wherein the obtained lower carboxylic acid has one more carbon atom than the lower alcohol has. The process maintains at least a limited amount of water, and (a) an effective amount of a catalyst stabilizer (an iodide such as lithium iodide), (b) that of an iodide derivative of a lower hydrocarbon, and (c) that of an ester of the alcohol with the carboxylic acid, in the reaction medium. Japanese Patent Application Laid-Open No. 155147/1985 (JP-60-155147A) (Patent Document 3) discloses a process for producing an organic carboxylic acid which comprises allowing an alcohol to catalytically react with carbon monoxide in the presence of a homogeneous catalyst system composed of (a) an organic ester and (b) a rhodium metal atom and a mixture of lithium iodide and methyl iodide.
Moreover, Japanese Patent Publication No. 5839/1996 (JP-8-5839B) (Patent Document 4) discloses a process for producing a lower carboxylic acid which comprises allowing a feed composed of an alcohol essentially having one less carbon atom than the carboxylic acid has to react with carbon monoxide in a carbonylation reactor holding a liquid reaction medium containing a rhodium catalyst, wherein (a) an effective amount of a catalyst stabilizer (an iodide), (b) that of an iodide derivative of a lower hydrocarbon corresponding to the alcohol, (c) that of an ester of the carboxylic acid and the alcohol, and (d) at least 4 psi of hydrogen partial pressure in the reaction condition, as well as at least a limited amount of water are maintained in the reaction medium during the reaction. The patent document 4 mentions that increase in the hydrogen partial pressure enlarges the ratio Rh(I)/Rh(III) in the carbonylation reactor by a water-gas shift reaction, resulting in increase of a formation rate of acetic acid. Further, in the patent document 4, since the reaction rate significantly decreases at a water content of not more than 10% by weight in the reaction fluid, the reaction rate is increased by adding 5 to 30% by weight of lithium iodide. In such a process, however, since the proportion of Rh(I) relative to Rh(III) increases by keeping the hydrogen partial pressure of the reaction system not less than a certain pressure, the catalyst activity can be maintained to some extent. However, the hydrogen partial pressure is enhanced by supplying hydrogen to the reactor, and therefore generation of by-products (e.g., propionic acid, formic acid, and a hydrocarbon) increases even when the water content in reaction fluid is reduced. Further, in order to improve the space time yield, it is necessary to enhance the reaction pressure in the reactor or resupply carbon monoxide to a bottom solution of the reactor.                Patent Document 1: JP-60-54334A (claim 1)        Patent Document 2: JP-60-239434A (claim 1)        Patent Document 3: JP-60-155147A (claim 1)        Patent Document 4: JP-8-5839B (claim 1, FIGS. 3 and 4)        